I'm deliberately running a fast advancing game, but I don't think they're advancing too fast. Once a week (more or less) gaming, 3-4 hours per session, and they're half-way through 9th level after a year of playing.
I deliberately set most encounters at or very near their level, and use alot of templates and classed NPCs & monsters. I designate (but don't make public to the players) each dungeon as a 1, 2, or 3 level-gain dungeon (haven't done any 3-levellers yet), and then figure out encounters to match. I've not come -anywhere- near an 8-level dungeon, and my mind boggles at the thought. That's, what? 104 separate encounters at your CR? Yikes.
One thing I have noticed -- prestige classes don't mean much until 9th or 10th level. Related to that; one simply has more toys to play with at higher levels. If lower level characters had more choices and options than higher level characters, I'd be more likely to keep the characters at a lower level. As it is, being lower level simply means there are large parts of the PH they never get to use, and large parts of the DMG & MM I never get to use. It'd be very frustrating to look at that stuff and think "Gee, just another 7 years...if my character doesn't die!"
I don't think a 1st level party facing some ordinary orcs is somehow more rewarding, dangerous, or character-building than a 10th level party facing a group of higher-level orcish fighters with sorcerous backup (and if orc sorcerers aren't your thing, substitute any other CR10 encounter). DMed correctly, they should be just as rewarding, dangerous, and character building. If your high level play is all about killing bigger monsters, it's not the level that's at fault. It's the game.
Cheers
Nell.